Examples of prestack depth migration in TI media

Robert J. Ferguson and Gary F. Margrave

ABSTRACT

Wave field extrapolation by nonstationary phase shift can be formulated to allow velocity variation with both wavenumber and position, and so can accommodate anisotropic effects in highly inhomogeneous media. The examples presented use physical data, acquired at The University of Calgary in the Department of Geology and Geophysics modeling facility. The first represents a zero-offset recording in which the target geology is a reef edge underlying an anisotropic medium with a non-vertical axis of symmetry. Isotropic migration incorrectly positions the reef edge by 350 meters laterally. Anisotropic migration correctly positions the reef.

The second example is an anisotropic thrust sheet embedded in an isotropic medium. The sheet is made up of 4 separate blocks of a common anisotropic material with different axes of TI symmetry. The model is anisotropic and strongly heterogeneous. Two seismic lines were acquired; a constant source/geophone geometry (approximately zero offset), and a prestack geometry (split spread). Using isotropic migration neither data set could correctly image the model - especially the base of the model and reflections internal to the anisotropic thrust. Using anisotropic migration by nonstationary phase shift both data sets correctly image the model.

The extra cost in computer run time relative to isotropic migration is about 20%. Distribution of the TI thrust migration (prestack) over a number of computers reduced the run time by a factor proportional to the number of PCs.

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